California/Winged Floater

Length Up to 9.5 cm. Smaller than other western Anodonta species, the shell is elliptical and more circular than oval and fairly flat. The outer portion of the shell is yellowish-green, yellowish-brown, or brown, the interior (nacre) is white or bluish. The margin of the rear top of the shell is compressed and raised to form a high prominent wing (though some individuals may be wingless). The valves are very thin but slightly inflated (can be compressed in some individuals). The beak of the winged floater is flattened, not usually raised above the hinge line and contoured with up to 20 irregular single or double-looped concentric ridges.  A key lifecycle feature of this and other freshwater mussel species is the need for a host (usually a fish species) to carry the larvae (glochidia). Floaters do not appear to be host specific and in most cases the host species is unknown. Larvae are relatively large, and attach themselves to the fins or gills of their host fish with hook-like projections on each valve. They remain attached to a fish for several weeks (depending on species and water temperature) before letting go and dropping to the bottom where they burrow into the sediment. All Anodonta species have thin fragile shells compared to most other native mussels. This enables them to ‘float’ on less solid substrates like silt and mud. They have also earned their name from colony die-offs that occur from seasonal low oxygen and temperature stress in the summer. The post mortem build-up of gases in the shell cavity may float the light shells to the water’s surface.